Extrude edges (or a curve) using an offset/inset to create a mitred patch of faces?
Start with your shape:

Select all the edges and extrude along the Z axis:

Select all the faces, right click and choose &quot;Extrude Faces Along Normals&quot;. You can drag the mouse or enter a numerical value - if you want it inwards, use a negative value (ex: -0.5).

After extruding along normals, make sure to check &quot;offset even&quot; in the operator panel.

Next, select the whole mesh and flatten it by scaling to 0 on the Z axis (S + Z + 0).
This will leave some duplicate vertices (as the top and bottom faces are now &quot;flattened&quot; on top of each other). Select the whole mesh, and pick Mesh &gt; Clean Up &gt; Merge By Distance to clean it up.

Now you have your shape: